The invention relates to trifluoromethylene compounds of the formula I ##STR2## in which
R.sup.1 is an alkyl or alkenyl radical having 1 to 15 C atoms which is in each case unsubstituted, monosubstituted by CN or CF.sub.3 or at least monosubstituted by halogen, it also being possible in these radicals for one or more CH, groups, in each case independently of one another, to be replaced by --O--, ##STR3## --CO--, --CO--O--, --O--CO-- or --O--CO--O-- in a manner such that O atoms are not linked directly to one another,
A.sup.1 is a
(a) trans-1,4-cyclohexylene radical in which, in addition, one or more non-adjacent CH.sub.2 groups may be replaced by --O-- and/or --S--,
(b) 1,4-phenylene radical, in which, in addition, one or two CH groups may be replaced by N,
(c) radical from the group comprising 1,4-cyclohexenylene, 1,3-cyclobutylene, 1,4-bicyclo(2,2,2)octylene, piperidine-1, 4-diyl, naphthalene-2, 6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl, where the radicals (a) and (b) may be substituted by CN or fluorine,
Z.sup.1 is --CO--O--, --O--CO--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --C.tbd.C-- or a single bond and
m is 1, 2 or 3.
The invention furthermore relates to the use of these compounds as components of liquid-crystalline media, and to liquid-crystal and electrooptical display elements which contain the liquid-crystalline media according to the invention.
The compounds of the formula I can be used as components of liquid-crystalline media, in particular for displays based on the principle of the twisted cell, including highly twisted variants thereof, such as, for example, supertwist nematic (STN) or supertwisted birefringence effect (SBE), the guest-host effect, the effect of deformation of aligned phases or the effect of dynamic scattering.
Similar compounds having liquid crystalline properties and one terminal OCHF.sub.2 group have already been disclosed and crystal structure studies have been carried out on appropriate compounds [S. V. Sereda et al. in Kristallografiya, 32 (5), 1165 (1987) and ibid. 33 (1) 118 (1988)]. In contrast to the compounds according to the invention, however, these compounds contain nitrogen-containing bridging members. There are similar compounds having liquid-crystalline properties wherein a CO-CF.sub.3 group is linked to an aromatic 1,4-phenylene group described in U.S. Pat. No. 5,045,229. However, they generally have comparatively disadvantageous values for the optic anisotropy, are not as stable against UV-irradiation and are significantly inferior to the compounds according to the invention for modern display applications.